1. Field of the Invention
The present invention relates to the conversion of chemical energy to electrical energy, and more particularly, to an alkali metal electrochemical cell having a positive electrode preferably comprising a transition metal oxide calendered into electrode blanks or plates for subsequent processing into an electrode.
2. Prior Art
Related U.S. Pat. Nos. 5,435,874 and 5,571,640, both to Takeuchi et al., which are assigned to the assignee of the present invention and incorporated herein by reference, are directed to a process for manufacturing a freestanding sheet of cathode material. The process involves first adjusting the particle size of the cathode active material to a useful size, followed by mixing with binder and conductivity enhancing additives suspended in a solvent to form a paste. The paste is fed into a series of roll mills that calender it into the freestanding sheet having a thickness in the range of about 0.004 inches to about 0.020 inches. Alternatively, the paste is first pelletized before being subjected to the calendering step. The resulting cathode sheet material is dried and punched into blanks that are subsequently contacted to a current collector to form an electrode.
The step of calendering or compacting the paste or, alternatively, calendaring the pelletized cathode active material, is performed by two to four calender mills that sequentially press the active admixture into the freestanding sheet in the form of a tape. Sequential calendering is performed by running the tape from one roll mill to the next with the roll mills contacting the cathode active material along a single direction of travel or direction of contact. Electrode blanks are then punched from the freestanding sheet.
The preferred cathode active materials described in the Takeuchi et al. patents are silver vanadium oxide (SVO) and copper silver vanadium oxide (CSVO). These materials are coupled with a lithium anode and activated by a nonaqueous electrolyte to provide a cell. Such high energy density cells are particularly useful as power sources for implantable medical devices, such as cardiac defibrillators.
U.S. Pat. No. 6,174,622 to Thiebolt, III et al., which is assigned to the assignee of the present invention and incorporated herein by reference, describes the production of low basis weight electrode structures by subjecting an electrode active mixture to a secondary calendering step performed in a direction reverse or orthogonal to that used to form the initial sheet tape. Calendaring in an orthogonal or reverse direction fibrillates the fluoro-polymeric binder in other than the initial direction. This lets the binder spread in transverse directions to lower the basis weight of the electrode active structures or blanks in comparison to electrode blanks produced by the previously discussed Takeuchi et al. patents.
However, there is a need to further lower the basis weight of electrode structures incorporated into high energy density electrochemical cells, such as of the type having lithium as an anode active material. Lowering basis weight is important for increasing the surface area of the electrode admixture. A greater electrode active surface area means that thinner electrodes can be produced, which results in more electrode plates per unit volume for an electrochemical cell. It is believed that a greater surface area results in improved discharge performance, such as increased energy density and rate capability and greater cycling efficiency for rechargeable cells.